Volatile contaminants (VCs) of various forms are often present in waste water from, e.g., industrial sources in process or cooling water used in industrial operations, and in drinking water supplies. The VCs may be introduced into the water from a wide variety of sources, such as chemical processes, cleaning processes, crude oil and natural gas production operations, leaking storage tanks, and surface spills. The problems associated with the presence of VCs in water are wide spread, and the need for effective and efficient removal of such contaminants is increasingly recognized.
The Federal Clean Air Act, as amended in 1990, required the U.S. Environmental Protection Agency to develop and implement National Emission Standards for Hazardous Air Pollutants (NESHAP), and the EPA has promulgated regulations establishing emission limits for over 110 chemicals. Those regulations set forth strict limitations on emissions of listed chemicals from waste water into the environment.
Various methods of and apparatus for removing VCs from water and other liquids have been known and used in the prior art for a number of years. One of the traditional approaches, generally referred to as "air stripping", removes VCs from a contaminated liquid by passing a stream of clean air or other gas through the water so that VCs transfer from the liquid to the gas and may be removed from the system with the exiting gas. The mass transfer of VCs from liquid to gas proceeds in accordance with, and is generally described by, Henry's Law, which states that the partial pressure, and thus the concentration, of a volatile compound in a volume of gas in interfacial contact with a dilute solution of that compound in water is directly proportional to the concentration of the compound in the solution. That relationship can be mathematically expressed by the following equation: EQU Y.sub.A =(H.sub.C /P.sub.T)(X.sub.A)
Where
Y.sub.A =the mole fraction of compound A in air; PA1 H.sub.c =Henry's Constant for compound A, in atmospheres/mole fraction; PA1 X.sub.A =the mole fraction of compound A in solution; and PA1 P.sub.T =total pressure of the reaction system, in atmospheres. PA1 N.sub.A =molar flux (moles/L.sup.2 t) (L=length; t=time) PA1 D.sub.AB =binary diffusivity for system A-B (L.sup.2 /t) PA1 C.sub.AO =the interfacial concentration of A in the liquid phase, which is assumed to be at equilibrium with the gas phase at the interface (moles/L.sup.3) PA1 a=film thickness (L) PA1 r=C.sub.a /C.sub.AO PA1 Ca=concentration in the main body of the liquid.
So long as the concentration of a particular contaminant in the water is higher than the equivalent concentration of that contaminant in a volume of gas in interfacial contact with the water, transfer of molecules of the contaminant from water to gas will occur. Once the concentration of the contaminant in the water and the concentration in a volume of gas have reached equilibrium, no further net transfer will occur. Conversely, contaminant transfer from gas to liquid will occur across a gas-liquid interface when the concentration of the contaminant in the gas is above equilibrium in comparison to the concentration in the liquid, and such transfer will continue until equilibrium is reached.
It is generally understood that the mass transfer of a volatile component from a volume of liquid to a volume of gas across an interfacial contact area is not instantaneous, but is subject to various limiting factors including the rate of transfer across the interface, the rate of diffusion of component molecules through the liquid to the interface, and the rate of diffusion of component molecules through the gas from the interface. In most cases of interest (i.e., low concentrations of the volatile component in water) the rate of diffusion of the volatile component through the liquid is the most significant factor, and volatile component transfer from liquid to gas is favored by maximizing the interfacial area relative to liquid and gas volumes and by minimizing the distance of diffusion through the liquid to the interface.
The dynamics of mass transfer across a gas-liquid interface can be quantified. The rate of transfer of a compound at the gas-liquid interface has been derived by Bird, Stewart, and Lightfoot, and expressed as EQU N.sub.A =(D.sub.AB C.sub.AO /.alpha.)(1-r)
Where
The foregoing equation is taken from the book Transport Phenomena, R. Byron Bird, Warren E. Stewart, and Edwin N. Lightfoot, John Wiley & Sons, Inc., 1960 (at page 535).
Traditional methods of air stripping and steam stripping VCs from water include the use of simple aerated tanks, spray towers, bubble tray columns, and packed columns to create an gas-water interface. While these traditional methods and associated apparatus do achieve contaminant transfer and thus some VC removal from the water, such traditional approaches are very inefficient, requiring long processing times and high equipment volumes. The inefficiency associated with the traditional prior art approaches arises largely from the relatively low ratios of gas-water interfacial area to volumes provided by the equipment, and the relatively long liquid diffusion distances to an interface.
It has been suggested that improved VC removal performance may be achieved through the use of an air sparged hydrocyclone similar to designs used in the mineral processing industry for separation of solid particles from an aqueous suspension. Examples of particle separation methods and apparatus may be found in U.S. Pat. Nos. 4,279,743; 4,397,741; 4,399,027; 4,744,890; 4,838,434; and 4,997,549. In a 1993 paper ("A Novel High-Capacity Technology for Removing Volatile Organic Contaminants From Water", Proceedings of Waste Stream Minimization and Utilization Innovative Concepts--An Experimental Technology Exchange, Ye Yi, April, 1993) an air sparging process and apparatus was proposed. More specifically, the paper disclosed a continuous process in which contaminated water is introduced into the interior of a porous tube in a swirl flow pattern and air is introduced through the porous tube into the water flow. The porous tube is disposed in a vertical orientation and the contaminated water is pumped into the tube at the top and allowed to swirl around the inner wall of the tube to the bottom, while air is forced through the tube into contact with the water. The air to water flow ratio disclosed and used by Yi is two (2).
Yi further teaches that it will be necessary for the water be recycled through the apparatus a number of times to achieve significant VC reduction. In the Yi example, the contaminated water was recycled six times to achieve a reduction in benzene concentration from 150 ppm to 10 ppm.
Although the method suggested by Yi indicated a potential for effective VC removal, current environmental regulations and pollution prevention concerns demand greater reductions in contaminant concentrations, and process economics demand higher efficiencies than the possibilities recognized by Yi. The process parameters taught by Yi and the apparatus designs taught by the particle separation reference patents upon which the Yi teaching is based still fall short of solving the problems in removal of VCs from water and of addressing the needs of industry without the disadvantages of the prior art.